1. Field of the Invention
The present invention relates to a novel alumina stabilized by silica and retaining a high specific surface at high temperatures, to a process for the production thereof and to the use of such silica-stabilized alumina as a catalyst support material.
2. Description the Prior Art
More particularly for the treatment/conversion of exhaust gases emanating from internal combustion engines, it is known to this art to employ catalysts therefor whose support is based on alumina. It is also known to this art that the efficiency of a catalyst increases in direct proportion to the size of the contract surface between the catalyst and the reagents. To effect this desired result, it is necessary for the catalyst to be maintained in a maximum finely divided state, i.e., the solid catalytic particles constituting same must be as small and individualized as possible. Therefore, the fundamental function of the support is to maintain the catalyst particles or crystallites in contact with the reagents in the maximum finely divided state.
In light of the severe conditions to which exhaust gas treatment catalysts are subjected, their supports, on which precious metals can be deposited, must display excellent thermal stability, i.e., must retain a high specific surface at high temperatures, in particular up to 1,200.degree. C.
The alumina normally used as a support for such catalysts undergoes an irreversible phase transformation into .alpha.-alumina under the influence of high temperatures (particularly equal to or above 1000.degree. C.), which reduces its specific surface to less than 10 m.sup.2 /g and effects a sintering of the catalytic phase. The catalyst deteriorates and loses a significant proportion of its efficiency.
It too is known to this art to add to the alumina various compounds for improving its thermal aging properties. Thus, it is known to impregnate a preshaped, calcined, alumina-based catalyst support with a rare earth metal nitrate solution (U.S. Pat. No. 4,061,594) or with a solution of a silicon compound. However, the aluminas obtained still have an inadequate specific surface at high temperature.